The SRM Color Guide may help if you can hold a sample in clear glass up to this card thing.
If you have a hydrometer or refractometer you could get the specific gravity (or Brix and then convert) of the cider. Since it is a gallon, I believe the SG would approximate the PPG. For ex, if you put exactly one pound of sugar into exactly one gallon of water you should get 1.046 SG. For sugar water, the last two digits (46) correspond to the PPG (potential points per gallon). Sugar is the benchmark because it gives up 100% of its weight to the sugar solution. Cider has some other stuff in it, so it should be less than 1.046. That should get you close. There might be information online, but I would think all ciders are not created equal.